The dynamin family of proteins consists of unique GTPases involved in membrane fission and fusion events throughout the cell. The founding member, dynamin, is crucial for endocytosis, synaptic membrane recycling, membrane trafficking within the cell and, more recently, has been associated with filamentous actin. Dynamin was first implicated in endocytosis when it was discovered to be the mammalian homologue of the shibire gene product in Drosophila. A temperature sensitive shibire allele causes a defect in clathrin-mediated endocytosis. Since then, overexpressing human dynamin mutants in mammalian cells was found to block clathrin-mediated endocytosis. Over the years, our structural work has played a leading role in dissecting the function of dynamin in membrane fission. We have shown that purified dynamin readily assembles into rings and spirals and it forms similar structures on liposomes, generating dynamin-lipid tubes that constrict upon GTP hydrolysis. The current model predicts dynamin wraps around the necks of coated pits and upon GTP hydrolysis constricts the necks and falls off leading to membrane fission. The ability of dynamin to constrict and generate a force on the underlying lipid bilayer makes it unique among GTPases as a mechanochemical enzyme. A potential mechanism for dynamin constriction was revealed when we solved the first three-dimensional structure of dynamin. We previously solved the structure of a dynamin mutant (lacking its C-terminus) in the constricted and non-constricted states using helical reconstruction and the IHRSR methods. The 3D volumes reveal three distinct radial densities, outer, middle and inner layers. During constriction the most obvious change is a decrease in the axial repeat and radius. However, the volume interior shows a large conformational change within the middle layer, which provides a clue to the mechanism of constriction. More recently, we have solved higher resolution (12 ) 3D map of &#916;PRD-dynamin. We have docked GMP-PCP GG domain (GTPase domain-GED fragment) crystal structure into our 3D map as well as the stalk domain from another dynamin family member, MxA, and the PH domain from dynamin. Based on the docking we predict the location of the dimer-dimer interface. Comparison between the GG domains in the GTP-bound and transition states, suggests that the conformational change induced by the GTP hydrolysis is driving a large swing of a 3-helical bundle near the GTPase core. We predict that the helical bundle movement is dynamins power stroke that results in a significant twist and constriction of the underlying lipid bilayer leading to membrane fission. During synaptic membrane retrieval, dynamin and endophilin are recruited to the necks of clathrin-coated pits and play a crucial role in vesiculation. Recently we have shown that endophilin and dynamin co-localize at the necks of clathrin-coated pits, proximal to the coat, after microinjection of GTP&#947;S. The interaction between dynamin and endophilin was further shown to be important for endocytosis by microinjection studies using the SH3-domain of endophilin and the synaptojanin derived peptide (PP19). Both perturbed the assembly of the proteins and inhibited endocytosis. In support of the role of endophilin enhancing the assembly of dynamin, we found that addition of endophilin increased the recruitment of dynamin to liposomes and the formation of tubules. These results suggest that endophilin and dynamin form a pre-fission complex at the necks of the coated pits proximal to the clathrin-coat, which coordinates dynamin-mediated budding of newly formed vesicles in vertebrate synapses.